This invention relates to cutting elements for use in rock bits and more specifically to cutting elements which include multiple diameter sections and to bits incorporating the same.
A cutting element, as for example a shear cutter as shown in FIG. 1, typically has a cylindrical cemented tungsten carbide body 10. The cylindrical body has a face forming an interface surface 12. An ultra hard material cutting layer 14 is formed over the interface surface 12. The ultra hard material layer is typically a polycrystalline diamond or polycrystalline cubic boron nitride layer. The ultra hard material layer typically has a planar upper surface 16 or a dome-shaped upper surface (not shown).
Shear cutters are generally mounted in pre-formed pockets 22 on a bit body 18 at a rake angle 20 typically in the order of 10°-20° (FIGS. 2 and 3). Each pocket has a rear support wall 23 which is a cylindrical section having a diameter slightly greater than the diameter of the cutter body. Typically a 90°-180° portion 24 of the cylindrical body outer surface 25 is brazed on the rear support wall. During drilling, the portion 27 of the cutting layer opposite the brazed area 26 is subjected to high impact loads which often lead to crack formations on the cutting layer as well as the delamination of the cutting layer from the cutter body. Moreover, these high impact loads tend to speed up the wear of the cutting layer. The component 138 of the impact load which is normal to the formations being drilled is a severe load because it is also reacting the weight of the bit body as well as the drill string. A majority of this load is reacted in shear along the interface between the cutting layer and the cutter body. This shear force promotes the delamination of the cutting layer from the cutter body.
To improve the fatigue, wear and impact resistance of the ultra hard material layer, i.e., the cutting layer, as well as to improve the ultra hard material layer's delamination resistance, it is common to increase the thickness of the ultra hard material layer, i.e., increase the volume of the material subject to impact during drilling. However, the increase in the thickness of the ultra hard material results in an increase in the magnitude of the residual stresses formed on the interface between the ultra hard material and the cutting element body which may result in early failure of the cutting element. Consequently, cutting elements are desired having improved ultra hard material layer fatigue, wear and impact strength, as well as improved delamination resistance.